This invention relates to radio communications systems, methods of coupling multiple transmitters to a single antenna, and specifically to a self-steering autoplexer for transmitter multicoupling.
The military has long sought a method for multicoupling many transmitters to a single antenna, ideally minimizing the cosite interference generated via some attribute of the multicoupling scheme as well as minimizing the mechanical structure and observability. Traditionally, manually tuned filters were combined into a single antenna as a multicoupler array, typically up to four to eight channels in HF, VHF or UHF bands. Operating such an array was an expensive, slow, operator-intensive, mechanically tuned process that was adequate when fixed-frequency operation was the norm. Today, however, the environment is much different, with a variety and increased quantity of channels needed for joint service and coalition communications, as well as the advent of fast frequency hopping systems and pressure to reduce crew size. The former increases the complexity of a multicoupling scheme due to the additional frequently-changing channels, and the latter the potential for interference, assuming that a frequency hopping multicoupler can be built at all.
New requirements for programs such as the US Navy Cluster M maritime and fixed site communications program are to multicouple on the largest ship on the order of 20 HF, 20 UHF and as many as 40 VHF transceivers to as few antennas as possible, and to simultaneously eliminate or minimize any interference among them. Those 80 channels may be any combination of hopping or non-hopping transmitters and receivers.
Current technical approaches for multicoupling these quantities of channels are typically implemented on a band-by-band basis due to the added complexity of multiband multicoupling. Current approaches include a hopping filter/multicoupler, a power bank amplifier, antenna isolation, and simple hybrid combining discussed in the following paragraphs.
The hopping filter/multicoupler approach is an electronically-tunable reincarnation of the traditional mechanically-tuned transmit multicoupler, using PIN diode tuned filters. Unfortunately, due to the low Q-factor of PIN diodes, these filters are very lossy, very large, and not very selective. The current four-channel FHMUX VHF SINCGARS multicoupler is an example of this technology.
A power bank amplifier is another alternative that linearly sums all the low-level transmit signals into a single large power amplifier, causing all the signals to emerge at once from one power amplifier output to be applied to one antenna. Unfortunately, there are two major flaws in this scheme. The low-level signals mix with each other via the non-linearity of the amplifier and generate a large number of intermodulation distortion products that are amplified and radiated together with the desired signals and that then interfere with many receive channels. The amount of power per channel is limited to 1/N^2, where N is the number of simultaneous signals. This is because the input signals add vectorally, and the maximum output power is limited to the peak vector sum of all signals, not the average power. This causes power division by the square of the number of signals and can be a very severe communications impediment when there are many signals.
The antenna isolation approach is a traditional method where separate antennas spaced apart are used. This is not an option when the goal is to reduce the number of antennas on an already small platform such as a ship.
The simple hybrid combining approach is another classical method of multicoupling by combining multiple transmitters through hybrids, which are nothing but RF transformers. This approach is simple, frequency hopping compatible, broadband, but inefficient since the combined signals are independent and non-coherent, each transmitter is effectively double-terminated by the hybrid, and hence half the power (−3 db) is lost as heat dissipation for each level of hybrid combining. A current shipboard HF system combines hybrid combining with a power bank amplifier resulting in a system that performs poorly because multiple signals and the hybrid losses result in so little power output per channel that the ships cannot communicate with anyone.
Currently there is no solution offered that results in acceptable transmitter multicoupling performance when accounting for wide bandwidths, multiple bands, frequency hopping, limited platform size, low observable antennas, and usable power output.